Lotus Elan

CD175 mixture adjustment

PostPost by: rdssdi » Sat Jul 07, 2012 12:53 pm

I have a 1969 Elan +2 with two Zenith Stromberg CD175 carbs. I replaced the original emissions air pistons and mixture needles with adjustable mixture pistons and needles. My car is running rich although the color tune shows a blue combustion. I understand blue can still be rich mixture.

I was told that I may have installed the needles improperly and possibly over adjusted them and that will separate the mixture needle from the adjuster and no longer move with turns of the tool. Further, I was informed that the set screw should be tightened and then backed off 1/4 turn.

Can anyone explain the proper assembly and adjustment? Will the jet and jet setting that was used on the earlier emissions set up be proper with the adjustable feature?


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PostPost by: oldelanman » Sat Jul 07, 2012 3:53 pm

Hi Bob,
The adjustable needle set-up uses a grub screw with a spring loaded plunger which just prevents the needle carrier from rotating and from coming right out of the air valve but does not clamp it in the air valve as it does in the fixed needle set-up. Unless the grub screw plunger is siezed it should not be necessary to back it off after tightening.

On my carbs total range of needle adjustment is only just over 3 turns - 1.5 turns either way from the nominal setting with the needle shoulder flush with the base of the air valve - so it is pretty easy to overdo it and disengage the adjuster thread from the needle carrier.

I made some notes on this when I had my carbs apart which you may find useful. I will PM them to you.
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